In recent years, the synthesis and application of bio-based fuels and platform chemicals have attracted growing interest. 5-Hydroxymethylfurfural (HMF) was recognized by the U.S. Department of Energy (DOE) as one of “Top 10+4” bio-based chemicals (Green Chem., 2010, 12, 539). This bio-based platform chemical can be prepared via hexose dehydration of carbohydrates. HMF could be upgraded facilely into various useful chemicals, due to the presence of active groups such as primary hydroxyl and formyl. HMF could be transformed into 2,5-dihydroxymethylfuran (DHMF), 2,5-dihydroxymethyltetrahydrofuran (DHMTHF), 2,5-dimethylfuran (DMF) and 2,5-dimethyltetrahydrofuran (DMTHF) through selective reduction, and their structures were shown in FIG. 1. These reduced products were important fuel additives as well as building blocks, which have broad applications in fuel, medicine and polymer industries. For example, DHMF is a versatile building block for the synthesis of drugs and crown ethers (J. Am. Chem. Soc., 1974, 96, 7159) as well as bio-based polymers with multi-shape memory and self-healing ability (Macromolecules, 2013, 46, 1794; ACS Appl. Mater. Interfaces, 2014, 6, 2753).
To date, DHMF was synthesized mainly via chemical methods. For example, Cottier et al. described DHMF synthesis via stoichiometric reduction of HMF using two equivalents of sodium borohydride at 4° C.; DHMF was achieved in a yield of 97% (Synth. Commun., 2003, 33, 4285). Alamillo et al. reported HMF reduction over CeOx-supported Ru at 130° C.; after 2 h, HMF was transformed completely, and DHMF was obtained in a yield of 81% (Green Chem., 2012, 14, 1413). Ohyama et al. reported a gold sub-nano cluster supported on Al2O3 catalyzed hydrogenation of HMF at 120° C. with 6.5 MPa H2, affording DHMF in the yield of 96% (RSC Adv., 2013, 3, 1033). Lin et al. described catalytic transfer hydrogenation of HMF to DHMF over low-cost ZrO(OH)2 at 150° C.; after 2.5 h, the conversion of HMF was up to 94%, and the selectivity toward DHMF was 89% (Green Chem. 2016, 18, 1080).
Although significant progress in the chemical synthesis of DHMF was achieved, chemical methods suffered from harsh reaction conditions, unsatisfactory selectivity, and use of toxic catalysts and organic solvents and stoichiometric reductants, etc. Recently, biocatalysis has received increasing attention in both industry and academia, because the problems described above can be overcome. Nonetheless, biocatalytic reduction of HMF to DHMF remains challenging, due to the following facts: (1) to continuously shift the reaction toward reduction, stoichiometric costly cofactors NAD(P)H or complex regeneration systems of cofactors were required in alcohol dehydrogenase-mediated reduction of HMF; (2) although using microbial whole cells as biocatalysts is well able to overcome the above problem of coenzyme recycling, the substrate HMF is a well-known potent inhibitor to microorganisms, which exerts strong inhibitory effects on microorganisms (Bioresour. Technol., 2000, 74, 25); as a result, microorganisms generally show poor tolerance to HMF, and low reaction rates and so on. Although the HMF transformations catalyzed by microbial cells have been reported, their main objective is to biologically detoxify lignocellulosic acid hydrolysates, where microorganisms transformed the inhibitor HMF present in lignocellulosic hydrolysates into the low-toxicity compounds (Appl. Microbiol. Biotechnol., 2004, 64, 125). However, these microorganisms could not meet the requirements of biocatalysts for efficient synthesis of DHMF from HMF, because of the following reasons: (1) their biodetoxification efficiencies remained low, suggesting that HMF reduction rates were low; for instance, Lopez et al. found that complete transformation of HMF of a low concentration (15 mM) using Coniochaeta ligniaria NRRL 30616 required the period of 70 h (Appl. Microbiol. Biotechnol., 2004, 64, 125). Zhang et al. reported a strain Enterobacter sp. FDS8 which exhibited high efficiency in HMF degradation; nonetheless, the HMF concentration tested (3.2 mM) was pretty low (Biochem. Eng. J., 2013, 72, 77). (2) Their tolerance to HMF, especially that of high concentrations, was poor (Biotechnol. Biofuels, 2014, 7, 51). According to the previous reports in the literature, HMF would exert a significantly deleterious effect on biotransformations when the concentrations of HMF were high (Biotechnol. Biofuels, 2015, 78, 63). (3) The selectivities were not satisfactory; for example, Feldman et al. reported that Pleurotus ostreatus could completely transform 30 mM HMF within 48 h, but the products contained both the reduced derivative DHMF and the oxidized derivative 2,5-furandicarboxylic acid (Biotechnol. Biofuels, 2015, 78, 63). Therefore, microorganisms that are tolerant to high concentrations of HMF and have high activities and selectivities are critical to constructing an efficient biocatalytic approach to DHMF from HMF.